The Andromeda Galaxy, also known as Messier 31, M31, or NGC 224, is a spiral galaxy approximately 780 kiloparsecs (2.5 million light-years) from Earth. It is the nearest major galaxy to the Milky Way. It received its name from the area of the sky in which it appears, the constellation of Andromeda, which was named after the mythological princess Andromeda.
Being approximately 220,000 light years across, Andromeda is the largest galaxy of the Local Group, which also contains the Milky Way, the Triangulum Galaxy, and about 44 other smaller galaxies. Despite earlier findings that suggested that the Milky Way contains more dark matter and could be the largest in the grouping, the 2006 observations by the Spitzer Space Telescope revealed that Andromeda contains one trillion stars - at least twice the number of stars in the Milky Way, which is estimated to be 200–400 billion.
The Milky Way and Andromeda are expected to collide in 3.75 billion years, eventually merging to form a giant elliptical galaxy or perhaps a large disc galaxy. The apparent magnitude of the Andromeda Galaxy, at 3.4, is among the brightest of the Messier objects, making it visible to the naked eye on moonless nights, even when viewed from areas with moderate light pollution.
Image Credit: Adam Evans via wikipedia.org
Explanation from: https://en.wikipedia.org/wiki/Andromeda_Galaxy